In recent years, a considerable effort has been made towards the synthesis of conjugated systems with low band-gaps[1]. A highly effective methodology for achieving low band gap materials is through the synthesis of donor-acceptor π-conjugated systems[2]. Donor-acceptor (D-A) π-conjugated systems contain alternating electron rich (donor) and electron deficient (acceptor) conjugated moieties incorporated into the main chain. The properties of such systems hold potential for application in organic field effect transistors (OFETs), organic light-emitting diodes (OLEDs) and photovoltaic cells (OPV)[3]. The synthetic flexibility of D-A polymers enables the modulation of the frontier orbital energy levels resulting in the ability to fine tune the material's properties[4]. A reduction in band gap can be achieved through enhancing the strength of donor and acceptor moieties as larger orbital interactions result in an increased double bond character and stabilisation of the quinoidal mesomer.[1] Whilst current donor-acceptor motifs are highly promising, there is still ongoing research into the modification of the donor and acceptor units to engineer materials that show ever improving performance.
One methodology is increasing the electron deficiency of the acceptor moiety by appending electron withdrawing groups, which reduces the band-gap by lowering the LUMO level[2]. Binding of a Lewis acid onto an acceptor moiety has been shown to be an effective way of significantly increasing the electron withdrawing power of the acceptor. This produces a considerable decrease in the energy of the LUMO, which is essential for developing high performance organic electronics with high electron affinities and mobilities[3].
Poverenov et. al.[4] recently demonstrated that benzothiadiazole (BT) containing D-A polymers could be doped by coordination of low amounts of a Lewis acid, which led to a dramatic increase in conductivity. However, the drawback of such Lewis acid binding strategies is that the resulting systems are highly sensitive, with the BT→Lewis acid dative bond being readily cleaved by both water and Lewis bases.
It is therefore desirable to develop stable D-A compounds having optimized conductivity characteristics.
The present invention was devised with the foregoing in mind.